1. Field of the Invention
The present invention relates to an electromagnetic shielding film and an optical filter, particularly to an electromagnetic shielding film and an optical filter that are light transmittable and capable of shielding electromagnetic waves from front surfaces of display devices such as CRTs (cathode ray tubes), PDPs (plasma display panels), liquid crystal displays, EL (electroluminescence) displays, and FEDs (field emission displays), microwave ovens, electronic devices, printed circuit boards, and the like.
2. Description of the Related Art
Problems of electro-magnetic interference (EMI) have been rapidly increasing in recent years along with increase in the use of various electric and applied-electronic equipments. It has been indicated that the EMI causes malfunction or failure of the electronic or electric equipment, and further causes health disorder of an operator of the equipment. Therefore, the electronic and electric equipments are required to reduce the intensity of an electromagnetic wave therefrom to make it within a standard or a regulation.
The electromagnetic wave has to be shielded to solve the EMI problems, and a metal having a non-transmissivity to the electromagnetic wave can be utilized for the shielding obviously. For example, the electromagnetic wave can be shielded by a method of using a housing of a metal or a highly conductive material, a method of inserting a metal plate between circuit boards, a method of covering a cable with a metal foil, etc. Display devices such as CRTs and PDPs need to be light transmittable such that the operator can recognize characters shown on the screen. However, in the case of using the above methods, the front surface of the display device is likely to be opaque. Thus, the methods are unsuitable for shielding the electromagnetic wave from the display devices.
Particularly the PDPs generate a larger amount of the electromagnetic wave as compared with the CRTs or the like, whereby electromagnetic shielding materials for the PDPs are required to have a higher electromagnetic shielding ability. The electromagnetic shielding ability can be simply represented by the surface resistance. The light transmittable, electromagnetic shielding materials for the PDPs need to have a surface resistance of 2.5 Ω/sq or less, while those for the CRTs need to have a surface resistance of about 300 Ω/sq or less. In a consumer plasma television using the PDP, the electromagnetic shielding material is required to have a remarkably high conductivity with a surface resistance of 1.5 Ω/sq or less, more desirably 0.1 Ω/sq or less.
The light transmittance of the electromagnetic shielding material is required to be about 70% or more in the CRT and about 80% or more in the PDP, and there is a demand for further improving the light transmittance.
Generally a film material produced by forming a metal mesh on a film is used as the electromagnetic shielding material for the plasma display panel (PDP) having excellent light transmittance and high electromagnetic shielding properties. However, a moire is often generated due to interference between the film material and a pixel pattern of the PDP.
Conventional fiber meshes have large wire widths to achieve high conductivity, and thereby the moire is easily generated (see Japanese Laid-Open Patent Publication No. 5-327274, etc.) The moire can be effectively reduced by thinning a wire of the mesh pattern. For example, a thin wire having a width of about 10 μm obtained by etching a copper foil is described in Japanese Laid-Open Patent Publication No. 2003-046293. However, this method is disadvantageous in complicated production process and in high cost.
As a method for forming the mesh pattern with improved productivity, an electroless plating method, which contains printing fine metal particles as a catalyst and depositing a conductive metal on the printed fine metal particles, is disclosed in Japanese Laid-Open Patent Publication Nos. 11-170420, 2004-068119, and 2004-068120. Further, a mesh pattern forming method using a silver salt diffusion transfer process is proposed in WO 2004/007810 and Japanese Laid-Open Patent Publication No. 2004-172041. However, in these methods, when the wire width of the mesh pattern is reduced, the surface resistance representing the electromagnetic shielding property is increased. Further, these methods are poor in wire resolution. Thus, in these methods, the mesh pattern has a large wire width of 20 to 30 μm, so that the moire cannot be sufficiently reduced.
Further, a method of developing a silver halide to form a conductive mesh of metallic silver, and a method of plating the developed silver mesh with copper to form a conductive mesh are proposed in Japanese Laid-Open Patent Publication No. 2004-221564, etc.
The problem of the moire can be solved to some extent by optimizing the bias angle of the mesh for each of PDP pixel patterns. However, a large number of suitable films are needed for the PDP pixel patterns, whereby this method is disadvantageous in increased cost and storage. To reduce the number of the films, it is necessary to produce an electromagnetic shielding material having a wide acceptable angular range within which the moire is not generated by the PDP pixel pattern.
In the case of improving the productivity by selecting the mesh forming method, the surface resistance is increased by reducing the wire width, or the wire width cannot be reduced due to limitations of thin wire mesh resolution, so that the moire is often increased. Thus, there is a demand for a method for reducing the moire without thinning the metal wire in the mesh.
Under such circumstances, an object of the present invention is to provide an electromagnetic shielding film and an optical filter that are highly light transmittable and electromagnetic shieldable, and capable of minimizing image quality deterioration due to moire or the like.
In ordinary crossed mesh patterns, in the relation between the moire and the wire width, the moire is unnoticeable when the wire is thin, for example, at a wire width of 10 μm or less, whereas the moire is highly visible when the wire becomes thicker, for example, at a wire width of 15 μm or more. As described above, as the wire of the mesh is thickened (widened), the moire is increased. Though the moire can be effectively reduced by thinning the wire of the mesh pattern, the thinning results in surface resistance increase and complicated production process disadvantageously.